The first automatic action of current injection devices is to drive a syringe forward and push a needle of the syringe into the patient. This is achieved by applying a force to either the syringe body (“direct drive”) or the syringe piston (“piston drive”). Piston drive is a simpler approach to engineer as one mechanism can be used to drive the needle out and then expel the contents of the syringe. However, this approach does carry a risk that a small quantity of the fluid may be expelled before the needle has reached a required depth in a patient's skin. The system relies on the needle reaching full depth before a significant quantity of fluid can be expelled. The rate of fluid delivery is limited by the force available, the viscosity of the fluid and the restriction of the needle. This property has sometimes been described as hydraulic lock.
It is therefore an aim of the present invention to provide an injection device utilising a piston drive approach which does not expel any fluid before a needle has reached a required depth in a patient's skin. As ever, the simplest and cheapest way of achieving this is sought.